The present invention relates to aircraft anti-collision warning devices and, more particularly, to devices for directing the full attention of an aircraft operator in a direction in which the presence of a potential collision threat is detected.
In the early days of aviation, aircraft were scarce, and the sky was thought to be very large. Collision avoidance was not considered to be a serious problem. Over time, many more aircraft have joined the commercial and general aviation fleets. Now, close approaches between aircraft are quite common. A mid-air collision between a commercial jet and a single-engine aircraft, and their subsequent 100-percent fatal crash, near San Diego, increased the interest in finding ways to avoid mid-air collisions.
The Federal Aviation Administration imposes flight restrictions around all of the high-traffic airports in the country to deny such airspace to most of the general aviation fleet, with the thought that this would avoid close encounters between general aviation and commercial aircraft. In such restricted airspace, all aircraft are in communication with, and under the control of, an air traffic controller, who has both primary radar (conventional radar which depends on radio waves reflected from the target to the radar) and secondary search radar (SSR) available to inform the controller of the position and altitude of all controlled aircraft. SSR employs an aircraft-mounted transponder to produce a radio signal in response to an interrogation signal transmitted by the ground radar. It is generally accepted that such restrictions were in place, and that both aircraft were directed by the FAA, during the San Diego incident noted above, but that the measures in place did not prevent the occurrence of a collision between the two aircraft.
Primary radar is a system in which radar energy is directed in a vertically broad and horizontally narrow beam by a rotating antenna. Radar energy, reflected from an aircraft target, is detected at the radar site. The azimuth angle of the target with respect to the radar antenna is determined by the direction in which the antenna is pointing when a reflection is received. The distance to the target is determined by the round-trip transit time of the radar energy from the antenna, to the target, and back to the antenna. Thus, a primary radar determines the X and Y positions of a target, but is incapable of determining target altitude (Z). Besides the unavailability of altitude information, primary radar suffers from variations in reflectivity of a particular target with changing aspect, and differences in target reflectivity between different targets. These differences create substantial differences in the brightness of the image (blip) painted on a radar screen.
The problems with primary radar was the principal genesis of the Air Traffic Control Radar Beacon System (ATCRBS), generally pronounced "at crabs". The ATCRBS system consists of a secondary search radar (SSR) antenna co-located with the primary radar antenna, and a beacon transponder in each aircraft. The SSR interrogation antenna, producing a vertically broad and horizontally narrow beam of energy at 1030 MHZ, rotates with the primary radar antenna. Beacon transponders in aircraft, when they receive the 1030 MHZ interrogation signal, produce a pulse-coded output signal at 1090 MHZ that contains either identity or altitude information. The identity information is dialed in from the front panel of the transponder, while the altitude information is derived on-board from an encoding altimeter. When an aircraft is operating under "positive control", that is, under control of a ground controller, the identity information dialed into the transponder is a four-digit code supplied by the ground controller. Thus, when the ground controller receives a coded identity code, this code serves to identify the aircraft.
A transponder transmits identity information (Mode A) in response to an identity interrogation signal, and altitude information (Mode C) in response to an altitude interrogation signal. An identity interrogation signal consists of a pair of 0.8 microsecond pulses spaced 8 microseconds apart. An altitude interrogation signal consists of a pair of 0.8 microsecond pulses spaced 21 microseconds apart. An SSR interrogation cycle includes two Mode A interrogations and one Mode C interrogation. One receiving the transponder coded signal, without further information, would not know whether the signal represents identity or altitude. The SSR sorts out Mode A from Mode C responses according to the nature of the just-transmitted interrogation signal. That is, if the SSR has just transmitted a Mode A interrogation signal, the SSR accepts a received coded signal as an identity signal. Alternatively, if the SSR has just transmitted a Mode C interrogation signal, the SSR accepts a received coded signal as an altitude signal.
The SSR signal, since it contains identity and altitude information, as well as azimuth and range, derived from antenna azimuth and transit time, has proven quite useful in air traffic control. A controller's radar scope, instead of displaying unidentified radar blips, is computer controlled to display selectively identity and altitude information of aircraft. The controller can choose to display only those aircraft in a particular altitude band (e.g. all aircraft between 5,000 and 12,000 feet) or only those in a particular area (e.g. the area of the controller's responsibility).
Even though all aircraft in a terminal area (generally known as Class B airspace) may be displayed on the controller's radar scope, experience has shown that the air traffic control system is ill prepared to assist pilots in collision avoidance. In many cases, controllers are understaffed for the present air traffic loads, particularly in large metropolitan areas. They are so hard pressed to adequately serve commercial traffic that they are, at times, unable to provide adequate protection of private, or general aviation, aircraft.
Attempts are under way to monitor courses leading to collisions using computer algorithms, and to issue warnings and/or instructions automatically based on the outcome of the algorithms. Such algorithms have proven imperfect and, in extreme cases, have directed pilots in directions leading to greater danger, rather than less. In addition, even when a controller issues a traffic alert, such an alert generally consists of a distance, a clock direction and a transponder-derived altitude (e.g. target at 5000 feet, four miles at nine o'clock). It is a common experience that pilots who receive such information fail to see the other aircraft, even though they are told the distance and relative direction of the target.
In the end, it is the responsibility of the pilot to avoid collision with other aircraft. This is true whether the aircraft is operated under Visual Flight Rules (VFR) or Instrument Flight Rules (IFR). Attempts have been made to develop on-board equipment to assist the pilot in detecting the presence of other aircraft in the vicinity of the pilot's aircraft. One such attempt employs passive listening to determine the azimuth direction and received signal strength of beacon transponders of all aircraft in the vicinity. Various display schemes are possible. In one display scheme, all received signals are displayed with their directions related to the center-line of the pilot's aircraft and their distances related to the received signal strength. This approach, although valuable, suffers from some admitted drawbacks. First, it lacks altitude information. Second, variations in signal strength between aircraft, and variations in signal propagation about an aircraft, make the strength-is-range concept imperfect.
A second approach, disclosed in my U.S. Pat. No. 5,223,847, the disclosure of which is herein incorporated by reference, stores three successive beacon responses from an aircraft. These responses are compared. Two of these responses are identity information, and one is altitude, since the SSR interrogates twice for identity while interrogating once for altitude. To decode the transmitted altitude signal, the coded signal which appears twice in the three successive responses is discarded, and the third, which must contain the altitude information, is decoded. This altitude is compared with the pilot's aircraft altitude to determine whether or not the two altitudes arc close enough to cause concern. In addition to determining the coded altitude transmitted by the other aircraft, the disclosed system includes a technique for determining the direction of the received signal. Combining the direction, and the comparison of altitude permits calculations that determine whether conditions exist that could lead to a collision. When a collision threat exists, a suitable display is energized.
Two problems are identified with the system of the '847 patent. First, encoding altimeter errors in the pilot's aircraft, and/or the target aircraft may indicate safe conditions, when, in fact, unsafe conditions exist, or vice versa. Second, even when the conditions are correctly received, a technique for directing the pilot's attention in the direction of the target (up/down and heading angle) is imperfect.
One approach for solving the altitude-error problem is disclosed in my U.S. patent application, Ser. No. 85,023 now U.S. Pat. No. 5,506,590, in which the content of the beacon transponder signal is ignored, and only the transponder signal itself is used to determine direction, heading angle and elevation angle of the transponder relative to the pilot's aircraft. An antenna array atop the aircraft, similar to the '087 patent, determines the direction and signal strength of a transponder signal. The content of the coded signal, both identification and altitude, are ignored--only the 0.45 microsecond framing pulses, spaced 20.4 microseconds apart, arc employed. An omnidirectional antenna at the bottom of the aircraft, together with the top antenna array, determines the vertical angle from which the beacon transponder signal originates. This assumes a reasonable spacing between top and bottom antennas of about 6 feet, which is customary for even small general aviation aircraft.
The system disclosed in the application indicates the direction of the origin of the beacon transponder signal within about 45 degrees (eight segments around 360 degrees). An indicator proposed in the application includes an array of three sets of eight LEDs arranged in a circle. Each set of LEDs contains a mini array of a green LED, a red LED and a yellow LED. If the signal strength indicates that the beacon transponder is within about 5 miles, and if its vertical angle is within about 7 degrees, a green LED is flashed in the selected sector. If the signal strength indicates that the beacon transponder is closer than about 5 miles, and more than about 2 miles, and is within an angular range of about 7 degrees above or below the aircraft, an amber LED flashes in the selected sector. Finally, if the signal strength indicates that the beacon transponder is within about two miles, and within an angular range of about 7 degrees above or below the aircraft, a red LED flashes. The LEDs flash at the rate at which ground interrogation reaches the target aircraft. This rate is about once every four seconds. The color of the flashing LED calls attention to its significance. The angle of 7 degrees is chosen because, at two miles range, this represents about 1000 feet vertical separation, which is considered to be a minimal amount for collision avoidance.
Using the techniques disclosed in my prior patent and application, it is also possible to determine, not only that a target is within an up-down angular range, but also whether the target is up or down with respect to the hull of the pilot's aircraft. This information can be communicated by illuminating a combination of LEDs, or by having two additional LEDs, one illuminated to indicate that the target is up, and the other illuminated to indicate that the target is down. If both are illuminated, the target is at approximately the same altitude, as derived from elevation angle.
A newer anticollision system called TCAS, not yet in widespread use, employs transponders in aircraft which interrogate each other at regular intervals of, for example, about once each second. TCAS transponders operate autonomously, rather than upon interrogation from the ground. The clutter in a busy terminal area from so many transponders asynchronously on the air at the same time makes it difficult to separately process anticollision signals. In addition, some TCAS systems reduce their transmitted signal strength in busy environments to reduce clutter. This interferes with strength-is-range type ranging but, due to the usual reciprocal relationship between signal strength and range, this type of ranging remains useful. A TCAS system can be recognized because, since the TCAS output occurs autonomously once per second, the appropriate LED is flashed at a once-per-second rate for each aircraft within the signal-strength and angle limits of the system, rather than at the rate of once every four seconds for responses to ground interrogation.
Even with the heading, up-down angular band and approximate distance to a target identified, a problem remains in communicating to the pilot the appropriate direction in which to direct the pilot's view. This is the same problem as attempting to locate visually a target identified by air traffic control as being "at 11 o'clock, 6500 feet, and traveling northeast". Indeed, the problem may be worse, since the disclosed LED indicators are generally mounted vertically the aircraft dashboard. It is a well-known difficulty to relate a direction presented in a vertical circle to a direction with respect to the fore-and-aft axis of an aircraft. This difficulty is compounded by the changing geometry of angles and line of sight between moving and maneuvering aircraft.